When it comes to determining your due date, "things," as the Gilbert and Sullivan ditty goes, "are seldom what they seem." The methods of calculation are far from exact, common assumptions about the average length of pregnancy are wrong and calling it a "due date" is misleading. Understanding these uncertainties may help to curb your natural impatience to know exactly when labor will begin.
Many obstetricians want to induce labor when you exceed your due date by a set number of days, in the belief that prolonged pregnancy increases risk. As with dating the pregnancy, the evidence for inducing labor after a certain time past the due date isn't nearly as clear-cut as you might think, but that's another subject.) If induction were harmless, it wouldn't matter, but it's not. Among other adverse effects, inducing labor increases the odds of fetal distress during labor and cesarean section in first-time mothers, and mistiming the induction can result in a premature baby.
How long does pregnancy really last?
You might be surprised how the idea of a 40-week pregnancy came into being. In the early 1800s a German obstetrician simply declared that pregnancy lasts ten moon months counting from the start of the menstrual cycle prior to the pregnancy. (2) It took nearly 200 years for researchers to investigate whether this was, in fact, true. It turns out that it wasn't. When researchers in the late 1980s followed a group of healthy, white women with regular menstrual cycles, they discovered that pregnancy in first-time mothers averaged eight days longer than this, or forty-one weeks plus one day (2). The average was three days longer than forty weeks in women with prior births. The researchers also refer to other studies suggesting that other races may have average pregnancy lengths that are shorter than white women.
As you can see, the due date was only a probability that labor would begin sometime around that day. It was not a certainty, much less a deadline. Until recently, obstetric practitioners defined a full-term pregnancy as extending anywhere from 37 to 42 weeks. Today, many obstetricians call any pregnancy lasting to the beginning of week 41 "postterm." This, you will note, is one day less than the average length of pregnancy in first-time mothers.